The Associated Society of Locomotive Engineers and Firemen or ASLEF is the British trades union for train drivers. Founded at Leeds in 1880 in emulation of the American Brotherhood of Locomotive Engineers which had been formed earlier in 1863. In all its 120 year history it appears to have neither changed its name nor merged with any other union.

It now has over 16,000 members employed by the variety of train operating companies that now run Britain's rail network.

The Union is affiliated to both the Trades Union Congress and to the Labour Party.